Medium- and high-power automatically tuned linear amplifiers

The essential features of any communications transmitter are reliability, ready means of frequency changing and linearity. Recently developed high-gain power valves require 1 kW, or less, driving power, which can be obtained from untuned wideband stages instead of the usual tuned drives. With only one tuned stage, it is easy to obtain a high degree of linearity, and automatic tuning by a frequency-following system becomes particularly attractive, without being unduly complex or expensive. A continuous frequency-following system with automatic loading control offsets changes in feeder impedance, so that the final amplifier is always operating under optimum conditions. This is a great advantage compared with preset systems. The design of compact, self-contained equipments, requiring no special under-roof ducts for air or cables, simplifies installation, particularly in existing buildings where new equipments replace obsolescent types. For new buildings the cost can be kept to a minimum. The paper details the electrical design principles for 7¿ and 30 kW automatically tuned linear amplifiers that contain wideband driving stages. The mechanical design is discussed and also the arrangement of suitable servo-tuning and rectifier circuits. Final specification figures and some typical test results are given.